This invention relates to presses for exerting a pressure over a longitudinal section of a sheet in general and more particularly to an improved press of this nature which permits obtaining a variable pressure profile across the sheet.
A press of this type having two opposed pressure plates and pressure elements is described in U.S. Pat. No. 3,881,852. Pressure elements are arranged alongside each other in rows transverse to the sheet form thereof of such elements. In travel direction of the sheet, a number of such sets corresponding to the length of the pressure section is also arranged one behind the other. The pressure elements belonging to a set extending across the sheet are connected with each other so that the sets pressure prevails in them and a correspondingly uniform pressure is transmitted to the sheet through the pressure plate.
It can happen that uneven pressures will occur in the sheet transverse to the latter. In the manufacture of wood chip board this can come about through uneven pouring of the chip mixture making up the sheet. This chip mixture then presents a different resistance to the compression when it is compressed to the same height. While such nonuniformities can be corrected by appropriate, careful pouring, there is another phenomenon which leads to a pressure profile across the sheet which cannot be influenced from the outside. This is the steam pressure which occurs in the interior of the sheet during the manufacture of wood chip board. For various reasons, the chip mixture must be adjusted to a certain moisture content. One reason is that, when the bed of chips runs into the hot press, steam must form in a sudden burst and then flow immediately into the interior of the mass of chips to cause rapid heating therein. Without this steam burst, there would be an excessive deal in heating the interior of the sheet being formed since heat can be supplied only from the surfaces.
This steam formation, which in itself is desirable, produces a pressure profile across the sheet with a high pressure in the center of the sheet and lower pressure toward the edges. This pressure distribution comes about because the steam can easily escape laterally at the edges, while more toward the center it must travel longer distances through the labyrinth-like mass of chips. The discharge resistance for the steam becomes larger toward the center of the sheet so that, in the center, the pressure building up due to the formation of the steam is better retained.
The pressure, which is nonuniform transverse to the sheet, now acts, among other things, on that pressure which is supported on the rear side by the pressure elements. These pressure elements provide only a uniform counter pressure. The uniformities in the sheet therefore lead to a flexural stress of the pressure plate. Because the occurring forces are substantial, due to the large areas, the flexural stress taking place can lead in some cases to appreciable deflections which result in undesirable nonuniformity of the thickness of the sheet.